1. Field
The present invention relates to a rotation detecting device adapted to be employed in association with, for example, an automobile anti-lock brake system (ABS) sensor, a wheel support bearing assembly utilizing such rotation detecting device and a method of making such rotation detecting device
2. Description of the Related Art
The technique has been suggested, in which a sensor and peripheral component parts are manufactured by means of an over molding (insert molding) of a resinous material. In an axle rotation sensor (an ABS sensor) used as fitted to a hub bearing assembly of an automotive vehicle, the structure is employed, in which a magnet body or a metallic body is arranged in a rotational ring of the hub bearing assembly and a magnetic sensor such as, for example, a magnetic pickup, a Hall sensor or a magnetoresistance element is arranged in a stationary ring in face-to-face relation therewith. The ABS sensor referred to above is used as a sensor unit structure with the sensor component parts over-molded.
In the conventional art, JP Laid-open Patent Publication No. 2000-88984, for example, discloses the over molding (secondary molding) of the resinous material with sensor component parts fixed to a sensor holder for fixing the sensor. In automotive vehicle component parts, such performances as the mechanical strength, the water proofing property, the weather resistance and the chemical resistance are required and, therefore, such a molding is necessary.
However, the conventional art involves the following problems.
(1) No adhesion between the covering material and the component parts built therein cannot be expected.
(2) As a result of change in ambient temperature and self-heating of electronic component parts built therein, a gap tends to occur between the covering material and the built-in component parts due to the difference in thermal expansion thereof, thus posing a problem in water proofing property.
(3) Even when an external force acts on the sensor unit molded and a plastic deformation occurs in the covering material, a gap tends to occur between the covering material and the built-in component parts, thus posing a problem in water proofing property.
(4) When an external force acts on the sensor unit molded, such force acts directly on the built-in component parts since the covering material comprised of a resinous material is less susceptible to deformation and, therefore, it will constitute a cause of breakage of the sensor unit.
(5) The covering material comprised of the resinous material lacks a vibration absorbing capability and, therefore, there is a problem in durability relative to external vibrations.
(6) With the molding by means of the conventional injection molding, a nozzle through which a molten resin flows, runners for leading the molten resin towards a cavity portion, which eventually forms a molded product, and inflow gates leading to the cavity portion are required. In order to smoothen the flow of the molten resin to thereby increase the yield, the appropriate number of pieces manufactured at one time is within the range of a few pieces to some tens pieces and thus, the number of pieces molded at one time is limited.
Also, the rotation detecting device such as, for example, the ABS sensor used as fitted to an automobile component part, particularly, the hub bearing assembly, is generally exposed to the road surface and is therefore wetted with a muddy salty water and is also placed under severe environment, in which a temperature change from some tens degree to minus some tens degree occurs. Yet, since the rotation detecting device is positioned below the suspension system, it is considerably affected by vibrations induced during the travel of the automotive vehicle. Any erroneous operation of the ABS sensor brings about a considerable influence on the vehicle traveling safety. For these reasons, lowering of the water proofing property resulting from an occurrence of a fore-mentioned gap must be avoided, which gap is caused by the poor adherence between the above described covering material and the built-in component parts and the difference in thermal expansion therebetween as well as the external force. Furthermore, even with respect to the durability against the vibrations and damage resulting from the external force, it has long been desired for an excellent one. Those keen demands cannot be fulfilled with the rotation detecting device formed by the conventional resin molding.